travis: build debian packages and documentation

The resulting packages are pushed to Debian repositories hosted on
bintray.com.

This is the first step to move away from our custom buildbot setup (see
https://i3wm.org/docs/buildbot.html for details on that) towards
infrastructure which is more standard (travis) and in the open.

travis/skip-pkg.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+# Returns true if Debian/Ubuntu packages should be skipped because this CI run
+# was triggered by a pull request.
+
+# Verify BINTRAY_USER is present (only set on github.com/i3/i3),
+# otherwise the CI run was triggered by a pull request.
+# Verify CC=gcc so that we only build packages once for each commit,
+# not twice (with gcc and clang).
+if [ ! -z "$BINTRAY_USER" ] && [ "$CC" = "gcc" ]
+then
+	exit 1
+fi
+
+exit 0